The present invention relates to processes for making polymeric multilayered films, and in particular to coextruded multilayered optical films having alternating polymeric layers with differing indices of refraction wherein at least one of the polymers is able to develop and maintain a large birefringence when stretched.
The present invention relates to processes for making polymeric multilayered films, and more particularly to coextruded multilayered optical films having alternating polymeric layers with differing indices of refraction. Various process have been devised for making multilayer film structures that have an ordered arrangement of layers of various materials having particular layer thicknesses. Exemplary of these structures are those which produce an optical or visual effect because of the interaction of contiguous layers of materials having different refractive indices and layer thicknesses.
Multilayer films have previously been made or suggested to be made by the use of complex coextrusion feedblocks alone, see, e.g., U.S. Pat. Nos. 3,773,882 and 3,884,606 to Schrenk, and the suggestion has been made to modify such a device to permit individual layer thickness control as described in U.S. Pat. No. 3,687,589 to Schrenk. Such modified feedblocks could be used to make a multilayer film with a desired layer thickness gradient or distribution of layer thicknesses. These devices are very difficult and costly to manufacture, and are limited in practical terms to making films of no more than about three hundred total layers. Moreover, these devices are complex to operate and not easily changed over from the manufacture of one film construction to another.
Multilayer films have also been made by a combination of a feedblock and one or more multipliers or interfacial surface generators in series, for example as described in U.S. Pat. Nos. 3,565,985 and 3,759,647 to Schrenk et al. Such a combination of a feedblock and interfacial surface generator is more generally applicable for producing a film having a large number of layers because of the greater flexibility or adaptability and lesser manufacturing costs associated with a feedblock/ISG combination. An improved ISG for making multilayer films having a prescribed layer thickness gradient in the thicknesses of layers of one or more materials from one major surface of the film to an opposing surface was described in U.S. Pat. Nos. 5,094,788 and 5,094,793 to Schrenk et al. Schrenk described a method and apparatus in which a first stream of discrete, overlapping layers is divided into a plurality of branch streams which are redirected or repositioned and individually symmetrically expanded and contracted, the resistance to flow and thus the flow rates of each of the branch streams are independently adjusted, and the branch streams are recombined in an overlapping relationship to form a second stream which has a greater number of discrete, overlapping layers distributed in the prescribed gradient. The second stream may be symmetrically expanded and contracted as well. Multilayer films made in this way are generally extremely sensitive to thickness changes, and it is characteristic of such films to exhibit streaks and spots of nonuniform color. Further, the reflectivity of such films is highly dependent on the angle of incidence of light impinging on the film. Films made with the materials and processes heretofore described are generally not practical for uses which require uniformity of reflectivity.
Several of the patents and applications discussed above contain teachings with respect to introducing layer thickness gradients into multilayer polymeric bodies. For example, U.S. Pat. No. 3,711,176 to Schrenk et al., teaches that it is desirable that a gradient or other distribution in the thicknesses of layers of one or more materials be established through the thickness of the film. Methods for creating gradients include embossing the film, selective cooling of the film during final stretching, and the use of a rotating die to create the layers as described in U.S. Pat. Nos. 3,195,865; 3,182,965; and 3,051,452. These techniques attempted to introduce layer thickness gradients into an already extruded film, and did not permit precise generation or control of the gradients. U.S. Pat. No. 3,687,589 to Schrenk et al teaches the use of a rotating or reciprocating shear producing means to vary the volume of material entering the feed slots of a coextrusion feedblock where the polymer streams are subdivided. Precise control of volumetric flow rates using such a device is difficult to achieve. In U.S. Pat. No. 5,094,788, Schrenk et al teach using variable vanes in an ISG downstream from a coextrusion die to introduce a layer thickness gradient into a multilayer polymer melt stream. U.S. Pat. No. 5,389,324 to Lewis et al describes control of the respective flow rates of the polymeric materials in the substreams to provide a differential in the volume of material flowing through each of the substreams. Because of the differential in the volume of the polymeric materials flowing in the substreams making up the composite stream, the individual layers in the body have a gradient in the thicknesses. The flow rate is controlled either by providing a temperature differential among at least some of the substreams, causing changes in the viscosities of the polymeric materials and thereby controlling their flow, or the flow rate is controlled by modifying the geometry of the passages or feed slots through which the plastified polymeric materials flow in the feedblock. In this way, the path lengths, widths, or heights of the substreams can be modified to control the flow rate of the polymer streams and thus the thickness of the layers formed.
To form a multilayered film, after exiting either a feedblock or a combined feedblock/ISG, a multilayered stream typically passes into an extrusion die which is constructed so that streamlined flow is maintained and the extruded product forms a multilayered film in which each layer is generally parallel to the major surface of adjacent layers. Such an extrusion device is described in U.S. Pat. No. 3,557,265 to Chisholm et al. One problem associated with microlayer extrusion technology has been flow instabilities which can occur when two or more polymers are simultaneously extruded through a die. Such instabilities may cause waviness and distortions at the polymer layer interfaces, and in severe cases, the layers may become intermixed and lose their separate identities, termed layer breakup. The importance of uniform layers, i.e., layers having no waviness, distortions, or intermixing, is paramount in applications where the optical properties of the multilayered article are used. Even modest instabilities in processing, resulting in layer breakup in as few as 1% of the layers, may severely detract from the reflectivity or appearance of an article. To form highly reflective bodies or films, the total number of layer interfaces must be increased, and as the number of layers in such articles is increased in the coextrusion apparatus, individual layer thicknesses become smaller so that the breakup of even a relatively few layers can cause substantial deterioration of the optical properties on the article. Problems of layer breakup are especially severe for multilayered bodies in which individual layer thicknesses approach about 10 xcexcm or less adjacent to the walls of the feedblock, multiplier, or extrusion die. Flow of multiple polymer layers through the feedblock and ISG typically entails both shear and extensional flow, while flow outside of the extrusion die is shear-free extensional flow. Layer breakup occurs inside flow channels very close to the channel walls where shear flow predominates, and is affected by such factors as small layer thickness, shear stress, interfacial tension between polymer layers, interfacial adhesion between the polymer melt and channel walls, and various combinations of these factors.
Several potential suggestions have been made to minimize flow instability, including increasing skin layer thickness nearest the die wall, decreasing the viscosity of the layer nearest the die wall by either increasing temperature or switching to a lower viscosity resin, reducing the total extrusion rate, or increasing the die gap. In U.S. Pat. No. 4,540,623 to Im et al, the use of sacrificial or integral skin layers on the order of from about 1 to 10 mils (25.4 to 254 xcexcm) is described to ease processing and to protect the surfaces from damage. These exterior skin layers are added immediately prior to the multilayer film exiting from the forming die or prior to layer multiplication. In U.S. Pat. No. 5,269,995 to Ramanathan et al, the use of protective boundary layers (PBLs) of a heat plastified extrudable thermoplastic material is taught to minimize layer instabilities. These layers may be internal to the multilayer body and/or on the external surfaces and generally serve to prevent layer breakup during the formation and manipulation of the multiple layers of polymers in a coextruded multilayer polymeric body.
While the previous discussion applies to multilayered films in general, often independent of the chemical, physical, or optical properties of the materials that make up the multilayered stack, by selective choice of materials and proper control of subsequent processing steps, multilayered films with enhanced optical or physical properties can be obtained. For example, U.S. Pat. Nos. 5,486,949 and 5,612,820 to Schrenk et al describe the use of birefringent polymers for the fabrication of coextruded polymeric multilayer optical films useful as interference polarizers. The birefringent polymers can be oriented by uniaxial or biaxial stretching to orient the polymer on a molecular level such as taught in U.S. Pat. No. 4,525,413 to Rogers et al. in order to obtain desired matches or mismatches of the in-plane refractive indices to reflect or transmit desired polarizations. Further, in U.S. patent application Ser. No. 08/402,041 to Jonza et al the use of birefringent materials useful for making interference polarizers and mirrors is described in which control of the relationships between the in-plane and out-of-plane indices of refraction gives coextruded polymeric multilayer optical films with improved optical properties at non-normal angles.
Recent developments in materials available for use in making polymeric multilayer optical films, and new uses for optical films which require improved control of layer thickness and/or the relationships between the in-plane and out-of-plane indices of refraction, have been identified. Processes described heretofore typically are not able to exploit the potential of the new resins available and do not provide the required degree of versatility and control over absolute layer thickness, layer thickness gradients, indices of refraction, orientation, and interlayer adhesion that is needed for the routine manufacture of many of these films. Accordingly, there exists a need in the art for an improved process for making coextruded polymeric multilayer optical films with greater versatility and enhanced control over several steps in the manufacturing process.
The present invention relates to methods and apparatuses for making multilayered optical films.
In brief summary, a useful feedblock useful for making a multilayer optical film of the invention comprises: (a) a gradient plate comprising at least first and second flow channels, wherein at least one of the flow channel has a cross-sectional area that changes from a first position to a second position along the flow channel; (b) a feeder tube plate having a first plurality of conduits in fluid communication with the first flow channel and a second plurality of conduits in fluid communication with the second flow channel, each conduit feeding its own respective slot die, each conduit having a first end and a second end, the first end of the conduits being in fluid communication with the flow channels, and the second end of the conduits being in fluid communication with the slot die; and (c) an axial rod heater located proximal to said conduits.
In brief summary, a method for making a multilayered optical film comprises the steps of: (a) providing at least a first and a second stream of resin; (b) dividing the first and the second streams into a plurality of layers using a feedblock comprising: (i) a gradient plate comprising first and second flow channels, where the first channel has a cross-sectional area that changes from a first position to a second position along the flow channel; (ii) a feeder tube plate having a first plurality of conduits in fluid communication with the first flow channel and a second plurality of conduits in fluid communication with the second flow channel, each conduit feeding its own respective slot die, each conduit having a first end and a second end, the first end of the conduits being in fluid communication with the flow channels, and the second end of the conduits being in fluid communication with the slot die; and (c) an axial rod heater located proximal to said conduits (c) passing the composite stream through an extrusion die to form a multilayer web in which each layer is generally parallel to the major surface of adjacent layers; and (d) casting the multilayer web onto a casting roll to form a cast multilayer film.
In brief summary, a method of making a textured multilayer optical film comprises the steps of: (a) providing at least a first and a second stream of resin; (b) dividing the first and the second streams into a plurality of layers such that the layers of the first stream are interleaved with the layers of the second stream to yield a composite stream; (c) passing the composite stream through an extrusion die to form a multilayer web in which each layer is generally parallel to the major surface of adjacent layers; (d) casting the multilayer web onto a casting roll; and (e) contacting the multilayer web by a micro-embossing roll to form a cast multilayer film.
In yet another method of making a multilayer optical film, the method comprises the steps of: (a) providing at least a first and a second stream of resin, wherein the first stream of resin is a copolymer of polyethylene naphthalate (coPEN) and the second stream of resin is polymethyl methacrylate (PMMA), (b) dividing the first and the second streams into a plurality of layers such that the layers of the first stream are interleaved with the layers of the second stream to yield a composite stream; (c) coextruding the composite stream through a die to form a multilayer web wherein each layer is generally parallel to the major surface of adjacent layers, wherein the coPEN and PMMA resins are coextruded at a melt temperature of about 260xc2x0 C., and wherein the birefringence of the coPEN resin is reduced by about 0.02 units or less compared to the birefringence of a homopolymer PEN resin for a given draw ratio; and (d) casting the multilayer web onto a casting roll to form a cast multilayer film.